1. Field
This invention relates in general to wireless communications. Specifically, this invention relates to systems and methods for direct conversion transceivers.
2. Background and Related Art
The field of communications has experienced a tremendous growth due in large part to the improved capabilities of wireless devices. Wireless devices employ radio waves to enable distant communications without the physical constraints of wire-based systems. Information, such as voice, data, or paging information, is conveyed by radio waves transmitted over predetermined frequency bands. Allocation of available frequency spectra is regulated to ensure that numerous users may communicate without undue interference.
Information to be transmitted from a source to a destination is seldom acquired in a format that is ready for radio transmission. Typically, a transmitter takes an input signal and formats it for transmission in a predetermined frequency band. The input signal, also referred to as a baseband signal, modulates a carrier in the desired frequency band. For example, a radio transmitter that receives an audio input signal modulates a carrier frequency with the input signal.
A corresponding remote receiver tuned to the same carrier frequency as the transmitter must receive and demodulate the transmitted signal. That is, the remote receiver must recover the baseband signal from the modulated carrier. The baseband signal may be directly presented to a user or may be further processed prior to being presented to the user. Many consumer wireless devices, such as radios, televisions, and pagers, are solely receivers.
Transceivers are wireless devices that integrate a transmitter and receiver in a single package. Transceivers enable nearly instantaneous two-way communications. Examples of transceivers include two-way radios, walkie-talkies, two-way pagers, and wireless phones.
Several figures-of-merit are important in assessing the effectiveness of a receiver design. Sensitivity determines the ability of a receiver to detect a weak signal. Receiver sensitivity must be such that the receiver can detect the minimal discernible signal (MDS) from background noise. Noise represents random fluctuations in voltage and current. The MDS a receiver-specific measure of sensitivity that incorporates the bandwidth of a given system. Receiver selectivity on the other hand, indicates the protection afforded a receiver from off-channel interference. The greater the selectivity, the better the receiver can reject unwanted signals.
Desense is a reduction in a receiver's overall sensitivity due to man-made or natural radio frequency interference (RFI). Desense occurs when a very strong interfering signal overloads the receiver and makes the detection of weaker signals more difficult. The desensitization characteristic of the receiver determines its ability to operate successfully under strong interferers, such as jammers.
The noise figure is another key measure of a receiver's performance. The noise figure degrades, that is, increases, at each successive stage in the receive path. Amplification or attenuation techniques may be applied within a receiver to achieve an acceptable noise figure. Noise, along with distortion, determines signal to noise and distortion (SINAD), a ratio in decibels which describes a receiver's performance in the presence of noise.
Distortion is the presence of unwanted signals at the output of devices in the RF path of a receiver. Distortion may include harmonic distortion, intermodulation distortion, and cross-modulation distortion. Harmonic distortion occurs when the desired input signal is large enough to compress the receiver and is typically measured at the baseband output as a function of frequency offset from the desired signal and as a function of the desired signal power. Crossover distortion occurs when the amplitude-modulated component from the transmitter (e.g., a CDMA wireless phone) is transferred to another carrier (jammer) at the output of the device (LNA output). The most common form of distortion is intermodulation distortion (IMD).
Intermodulation distortion is the result of two or more signals mixing together to produce additional unwanted distortion within the signal bandwidth. For two inputs, the intermodulation products occur at the sum and difference of integer multiples of the original frequencies. That is, for two input signals having frequencies f1 and f2, the output frequency components can be expressed as mf1±nf2, where m and n are integers ≧1. The order of the intermodulation product is the sum of m and n. “Two tone” third order components (2f1-f2 and 2f2-f1) can occur at frequencies near the desired or interfering signals and thus cannot be easily filtered. Higher order intermodulation products have lower amplitude; as such, they are less problematic. Second order intermodulation jamming products may be generated at baseband frequencies if the tone spacing is within half of the signal bandwidth.
FIG. 1 is a graph plotting the levels of fundamental, second order, and third order IMD components against input level. Theoretical points where the second order and third order levels intercept the fundamental are known as the second order intercept point (IP2 or SOI) and third order intercept point (IP3 or TOI). The IIP2 of a receiver is the input level second order intercept point. The IIP3 is the input level third order intercept point.
The third order intercept point and noise figure of a receiver are directly related to the receiver's dynamic range. The dynamic range defines the range of signals that the receiver can handle within the specified performance of the receiver, that is, the range over which the receiver can produce an accurate output with acceptable SINAD. Specifically, for a baseband receiver, such as an analog-to-digital converter, the dynamic range may be represented as spurious free dynamic range (SFDR), which ranges from the noise floor of the device to the maximum signal before clipping occurs.
Local oscillator (LO) leakage occurs when an LO signal leaks to the receiver input. Such leakage may be transmitted by the transceiver antenna as spurious emissions, which may interfere with other devices. In addition, LO leakage may be reflected back into the receiver itself and may desense the receiver if not removed prior to demodulation.
Jammer leakage occurs when a jammer signal leaks to an LO input or output of a device within a receiver. Such leakage may mix with the jammer signal to produce undesired signals, such as DC signal levels that are proportional to the amplitude modulation (AM) component of the jammer signal. AM jammer signals may be located at any frequency within a receive frequency band.
Low-frequency flicker (1/f) noise is caused by defects in the emitter-base junction of bipolar junction transistors. Although typically small, flicker noise and other such noise may need to be removed in a receiver in order to maintain signal integrity at baseband.
Isolation is the ratio (in dB) of the power level applied at one port of a device to the resulting power level at the same frequency appearing at another port. Reverse isolation, which is the inverse (reciprocal) of isolation, is a figure-of-merit for receiver components. Reverse isolation is a measure of how much energy injected into an output port makes it back into the input source. To achieve low LO and jammer leakage, high reverse isolation is desired.
The 1 dB compression point of an amplifier is a measure of the output power level when the amplifier gain is 1 dB lower than the small signal gain. The saturation point of an amplifier is a measure of the maximum output power capability of the amplifier. These figures-of-merit are illustrated in FIG. 1.
The above figures-of-merit and signal phenomena should be considered when designing wireless communication devices. More generally, the wireless communications landscape has been dominated by Code Division Multiple Access (CDMA), a form of spread spectrum, or broadband, communications in which radio signals are spread over a very wide bandwidth. CDMA technologies have been the basis for many modulation standards, such as CDMA (IS-95 and CDMA2000) and WCDMA (IMT2000). Each of these modulation or air-interface standards operates in many radio frequency bands, including Cellular (Japan Cellular and US Cellular), PCS (Personal Communications System in US and Korean bands), and IMT (International Telecommunications Union). Other modulation standards include FM (frequency modulation, IS-19), GSM (Global System for Mobile Communications), US-TDMA (IS-136), GPS (Global Positioning System), Wireless LAN (802.11), and Bluetooth.
Frequency bands have been allocated to various communications modes. For wireless transceivers, the US PCS receive (RX) frequency band is 1930-1990 MHz, and the associated transmit (TX) frequency band is 1850-1910 MHz. The US Cellular receive frequency band is 869-894 MHz, and the associated transmit frequency band is 824-849 MHz. Similarly, receive and transmit frequency bands are allocated to Japan Cellular, IMT, and Korean PCS.
Communications standards set forth specifications that wireless communication devices must meet. For instance, spurious emissions, sensitivity, jamming (two-tone intermodulation and single-tone desense), and residual sideband specifications must be met.
Wireless communications have not yet been standardized on an international, or even intranational, basis. Existing technologies have recognized that a transceiver that can operate in more than one band, or in more than one mode, has increased portability. In particular, dual band handsets operate on two frequency bands. For instance, a dual band CDMA handset can operate on both the 800 MHz (US Cellular) and 1.9 GHz (US PCS) frequency bands. If base stations operating on these two bands use the CDMA standard, then a mobile unit having a dual band CDMA handset may obtain service from either or both of these base stations. Further, a dual mode CDMA/FM handset may operate in both CDMA and FM modes. Yet, given the current multiplicity of modulation standards and associated frequency bands, dual mode and dual band phones offer subscribers at most a limited compatibility with communications systems of the world.
FIG. 2 is a high-level block diagram of a conventional dual downconversion receiver. Receiver 101 incorporates the super heterodyne architecture. In particular, a received RF signal 11 is conveyed along an RF signal path and preprocessed (stage 1). The preprocessed RF signal 13 is first translated, or downconverted, to a signal 15 having an intermediate frequency (IF) (stage 2). The IF signal 15 is then downconverted again to a baseband signal 17, which includes an “in-phase” (I) and “quadrature” (Q) phase component (stage 3). The I and Q baseband signal components vary in phase by 90°. The I and Q components are then sent to other portions of receiver 101, such as a baseband processor (stage 4), to be further processed. Similarly, in a dual upconversion transmitter, analog I and Q baseband signals are first upconverted to an IF signal, and the IF signal is then upconverted to a transmitted RF signal.
FIG. 3 illustrates receiver 101 in more detail. Receiver 101 has a number of inherent benefits. For example, the design offers excellent sensitivity and selectivity, an extended signal dynamic range, flexible frequency planning, and a lower dynamic range and current consumption for elements in receiver 101 after IF filters 70. In addition, phase and amplitude matching between the I and Q channels 106, 107 may be achieved more easily because the IF signal is at a lower frequency range. In view of these benefits, receiver 101 is well-suited for multi-mode and multi-band applications, wherein received RF signals—modulated in multiple modes and conveyed in multiple frequency bands—may be processed.
To support multiple bands and modes of operation, receiver 101 must include some mode-specific components. For instance, in a multi-band receiver, an individual RF signal path is typically required for each frequency band. In a multi-mode receiver, individual baseband paths may be required for each mode depending on jammer dynamic range requirements.
In conventional receivers such as receiver 101, the IF signal path typically includes amplifiers, filtering, and automatic gain control (AGC) circuitry. As such, receiver 101 can eliminate out-of-signal-band noise and jammers and can compensate for varying signal power and receiver gain changes. In a multi-mode receiver, filtering of IF signals is mode-specific. Therefore, receiver 101 has one IF filter 70 per mode. For instance, a receiver in a dual mode phone includes two IF SAWs (surface acoustic wave filter). For a receiver which supports the CDMA 1X, CDMA 3x, WCDMA, GSM, FM, Bluetooth, and GPS modes, four to six SAWs and 1 discrete LC filter may be required in the IF signal path.
The need for an IF filter for each mode is a significant drawback of receiver 101. Each IF filter increases the cost of the receiver, the number of critical parts, and the board area of the receiver. Because each IF filter may have high loss, an IF pre-amp or AGC may also be needed. An IF voltage controlled oscillator (VCO) and phase-locked loop (PLL) 65 are also needed to generate a local oscillator (LO) frequency, which is inputted to IF mixer 60. Additional drawbacks of receiver 101 include the need for a switch matrix or multiple IF amplifiers and AGC modules, the need for a low-loss RF bandpass filter (BPF) to reduce undesired sideband noise, and the need for additional IF mixers. Thus, the IF stage of a dual downconversion receiver increases cost, design complexity, and circuit board area of such receivers.
FIG. 4 is a block diagram of a direct downconversion, or zero IF, receiver 200. In direct downconversion receivers, a received RF signal 201 is directly downconverted to a baseband signal 225. Similarly, in a direct upconversion, or zero IF, transmitter, a baseband signal is directly upconverted to a transmitted RF signal. In receiver 200, the received RF signal is mixed with a local oscillator (LO) frequency to produce a baseband signal. Because it does not incorporate an IF signal path, receiver 200 eliminates cost, board area, and power consumption associated with IF components, which include IF SAWs, LC matching and discrete filters, a pre-amp, AGC, IF mixers, and the IF VCO and PLL. Further, less part-to-part and temperature variation occurs.
The design of receiver 200 allows for more signal processing, such as channel selectivity filtering, to occur in the baseband analog or digital domain via integrated circuits, thus enabling RF and analog portions of receiver 200 to be more generic in nature. Since the AGC is digital, simplified calibration, or even no calibration, may be required. For certain modes of operation, such as GPS, Bluetooth, and GSM, receiver 200 may not require an RF filter because a primary purpose of that filter is to reduce cross-modulation in CDMA Cellular and PCS modes. However, the GPS mode may require an RF filter if GPS-modulated signals are simultaneously received with other modulated signals.
Despite the above advantages, direct downconversion has not been widely incorporated into wireless phones. The reason is that it is very difficult to achieve key receiver design goals while achieving the proper dynamic range for the receiver. Design goals for receivers such as receiver 200 include achieving high gain and a low noise figure, high IIP3 and IIP2 values, and low power consumption. A multi-mode and multi-band receiver may require a very wide dynamic range. Accordingly, it is even more difficult to achieve these design goals for such a receiver.
More specifically, local oscillator (LO) leakage and jammer leakage into the I and Q mixer LO ports cause significant problems in direct downconversion receivers. For Cellular and PCS, the spurious emissions requirements are particularly stringent. As such, higher reverse isolation is needed. Additionally, in a direct downconversion receiver, LO leakage that is reflected back into the receiver itself, as well as jammer leakage to the LO port of the I and Q mixers, may be processed by the direct downconversion circuitry. As such, an undesired DC offset voltage may appear at the output of the mixer along with the desired baseband signal, which may also contain baseband spectral components. Accordingly, the DC offset must be removed to ensure that the signal-to-noise ratio is sufficiently high.
In CDMA, sensitivity is tested with a signal set to a level such that a certain frame error rate (FER) is met. IS-98 specifies that the device under test must meet a sensitivity level of −104 dBm (signal power) with less than 0.5% FER. The intermodulation test is conducted with a signal level set to −101 dBm (3 dB above the sensitivity test) with two tones at an offset relative to the RF signal (−43 dBm/tone at offsets that generate an in-band distortion product, or typically ±900 and ±1700 kHz) with less than 1% FER. Depending on the frequency band, there may be differences in the power levels tested and frequency offsets for the jammers. For the single-tone desense test, the jammer level at the RF port of the I and Q mixers is larger than the signal level by 71 dB at >=900 kHz offset.
The jammer power may leak to the LO port of each mixer and mix with the jammer level at the mixer RF port to produce a DC level that is proportional to the amplitude of the RF jammer. Typically, the jammer is generated by the forward link of a base station in a competing wireless system. The jammer power may change as a function of the modulation used or fading. The worst jammer may have amplitude modulation comparable to the desired signal bandwidth. As such, the AM component falls on top of any signal energy at baseband after downconversion and cannot be removed with baseband filtering. This problem is exacerbated as the jamming RF signal increases. If the jamming RF signal increases by 10 dB, for example, the baseband distortion increases by 20 dB. This baseband distortion can actually be greater than a two-for-one slope if both the RF to LO isolation of the RF mixers, which affects self-mixing of jammers, and the IIP2 of the RF mixers, which represents second order distortion effects, are poor.
Further, the jammer and LO leakage requirements for mixers in a direct downconversion receiver are very demanding. Because such a receiver lacks IF filtering, the dynamic range of the receiver baseband elements may need to be increased by 30 dB or more, depending on the degree of baseband analog filtering, and part-to-part, frequency, and temperature variations in gain. Residual sideband specifications for various modulation standards must also be met. Since such a receiver has less gain before its baseband stage, flicker noise at baseband has a greater effect on the ability of the receiver to process FM-modulated signals.
Therefore, what is needed is a direct conversion transceiver that can modulate RF signals in multiple bands and multiple modes.